Spark ignition of an internal combustion engine generally involves igniting an air/fuel mixture with an electric spark generated between a center electrode and a ground electrode of a spark plug. The facing surfaces of the center and ground electrodes are typically flat, and serve as arcing or firing surfaces between which the electric spark is generated. The voltage required to generate a spark between the electrodes is often referred to as the sparking, arcing, or demand voltage, and is known to be dependant in part on the spark gap between the electrodes and the size of the electrodes. Typically, the electrodes are formed from a nickel-based alloy which is resistant to the harsh electrical, thermal, chemical and mechanical environment of an engine's combustion chamber. The nickel-based alloy is often applied over a copper core which improves the thermal conductivity of the electrodes, such that excessive electrode temperatures are avoided that might otherwise cause auto-ignition.
As illustrated by U.S. Pat. No. 5,196,760 to Takamura et al., electrodes which are specially shaped to maintain the demand voltage at a relatively constant level over the life of the spark plug are known. Takamura et al. teach a center electrode having an annular undercut. The thickness of the portion above the undercut determines the useful life of the spark plug, in that the demand voltage remains generally stable while this portion exists, but increases drastically once the electrode has eroded down to the undercut. The portion of the electrode above the undercut has a circular cross-section, as is conventional, such that the circular perimeter of the electrode's firing surface provides a single and continuous sharp edge as an arc initiation site for an electric spark. However, as noted by Takamura et al., this edge is eventually consumed, such that suitable arc initiation sites available during the majority of the spark plug's life are limited to the electrode's firing surface and the sharp edge formed by the undercut and the portion remaining above the undercut.
As noted in Takamura et al., it is known to substitute a noble metal for the more conventional nickel alloys for the purpose of extending the life of the electrode. The use of a noble metal electrode is particularly advantageous when attempting to minimize the size of the electrode. As is known in the prior art, minimizing the size of an electrode reduces the potential for a phenomenon known as flame quenching or extinguishing, which occurs when an excessive amount of thermal energy in the flame kernel produced within the spark gap is absorbed by the electrode. As is also known in the prior art, a smaller electrode also serves to lower the demand voltage of its spark plug.
To minimize the amount of noble metal required, it is also known to attach a noble metal firing tip, such as a thin platinum alloy disc, to the firing surface of an otherwise conventional nickel alloy electrode, for the purpose of minimizing the amount of noble metal required. U.S. Pat. No. 4,700,103 to Yamaguchi et al. teaches a variation of this, in which a firing tip having minimal mass is welded to and projects from the surface of its base electrode. As a result, the firing tip is specifically configured to benefit from the advantages noted above with smaller electrodes. The firing tip taught by Yamaguchi et al. preferably has a rectangular shape with a square cross-section, with its longitudinal axis being oriented transverse to the axis of the center electrode. The firing tip is not coaxially aligned with the center electrode, as is conventionally done, but is offset such that only an edge of the firing tip is proximate to the center electrode. Alternative shapes are suggested for the firing tip, including circular, trapezoidal, and triangular cross-sections.
While the above approaches taught by the prior art generally achieve their respective objectives, further improvements in efficiency would be desirable. For example, though the demand voltage for the spark plug taught by Takamura et al. remains generally stable as long as the portion above the undercut is present, the demand voltage nevertheless increases significantly as the sharp edge at the firing surface's perimeter is eroded. While the noble metal firing tip taught by Yamaguchi et al. offers advantages over the electrode taught by Takamura et al., the potential arc initiation sites on the firing tip are generally limited to the proximate edge and its included corners. Consequently, as the spark gap increases with the consumption of the edge and its corners during the life of the spark plug, a corresponding and immediate increase in the demand voltage also occurs.
Therefore, it would be desirable if the advantages of a noble metal firing tip could be realized, in which the firing tip is specifically configured so as to achieve and maintain a relatively stable demand voltage over a longer period of time so as to further extend the service life of the spark plug. Such a firing tip would be even more desirable if an actual reduction in demand voltage could be achieved for a given spark gap. Finally, the firing tip would also preferably contribute minimal mass to a spark plug electrode so as to minimize the potential for flame quenching. A suitable method for forming such a firing tip would also be desirable in order to adapt the firing tip to mass production conditions.
The prior art spark plug 110 illustrated in FIGS. 1 and 2a-2g is representative of that taught by U.S. Pat. No. 4,700,103 to Yamaguchi et al. As is conventional, the spark plug 110 includes a ceramic body 112 which serves as an insulator between a center electrode 114 and a ground electrode 116. A first firing tip 118 is welded to the upper surface of the center electrode 114, while a second firing tip 120 is welded to the lower surface of the ground electrode 116. Both firing tips 118 and 120 are preferably formed from a noble metal, such as a platinum alloy. As can be seen in FIG. 1, the distal end 116a of the ground electrode 116 terminates short of the axis of the center electrode 114. The firing tip 120 projects from the distal end 116a of the ground electrode 116, such that an edge and two corresponding corners of the firing tip 120 are proximate the firing surface 118a of the firing tip 118. As such, the edge and corners serve as the firing "surface" 120a of the ground electrode 116, in that they represent arc initiation sites for the lowest resistance arcing path of an electric spark generated between the electrodes 114 and 116. As illustrated in FIG. 2a, the cross-section of the firing tip 120 shown in FIG. 1 is square, with the firing tip 120 being secured in a recess within the ground electrode 116. FIGS. 2b through 2g illustrate variations taught by Yamaguchi et al., in which the cross-section of the firing tip 120 is altered or the degree to which the firing tip 120 is recessed into the ground electrode 116 is altered.
While Yamaguchi et al. generally teach the advantages of spark plugs equipped with noble metal firing tips, a significant shortcoming of the spark plug 110 is that the firing surface 120a is not a surface at all, but only two corners of the firing tip 120 and their included edge. As such, the demand voltage for the spark plug 110 will increase significantly as the edge and corners erode, in that (1) the rate of erosion along the edge will be relatively high due to the spark being concentrated at a single edge, and (2) the spark gap between the firing tips 118 and 120 will increase as the firing surface 120a erodes. The alternative embodiments illustrated in FIGS. 2b through 2g provide, most preferably, two corners and one edge as arc initiation sites (FIGS. 2a, 2e and 2g) and, as a lesser preferred alternative, only one edge as an arc initiation site (FIG. 2c). Though three edges are present with the trapezoidal cross-section shown in FIG. 2e, the oblique edges are not preferential arc initiation sites due to their relative distance from the firing surface 118a on the center electrode 114. A further disadvantage with each of the configurations shown in FIGS. 2a through 2g is that, by locating the firing tip 120 within a recess in the ground electrode 116, the center and ground electrodes 114 and 116 are positioned closer together, thus increasing the potential for flame quenching.